3 Study Design 3.1 General Design We will recruit patients (18-75y) with excess glucocorticoids either because they have Cushing's syndrome or they have inflammatory conditions requiring GC treatment (e.g. rheumatoid arthritis, giant cell arteritis/polymyalgia rheumatica) into a pilot, randomised, double-blind, placebo-controlled trial. These patients will be treated with metformin to prevent or reverse their metabolic complications. Prevention algorithm: Patients who are about to start GC treatment predictably for ≥12w at a ≥10mg/d prednisolone (or equivalent) dose who consent to participate in this study will be randomly assigned to receive either placebo (20 patients/group, see power calculations) or metformin at the maximum tolerated dose with a minimum of 850 mg bd for 12w. Treatment algorithm: Consenting patients already on long-term GC treatment (≥4w, ≥20mg/d) who are expected to continue for at least 12w at ≥10mg/d prednisolone will be randomly assigned to receive either placebo or metformin for 12w. In both algorithms, metformin treatment will be started gradually (as standard practice) to avoid gastrointestinal side effects and the full dose will be reached by day 10. Patients will have a full clinical assessment before the start of the metformin treatment and at the end of the 12w treatment period. Anthropometric and biochemical parameters and questionnaires will be repeated at 4 and 8 weeks.